1. Field of the Invention
The present invention relates to a vehicle headlight equipped with a leveling device and a mechanically driven beam switching device.
2. Description of the Related Art
Vehicle headlights are adapted to project either a low beam or a high beam by causing the light from a light source bulb to be reflected in a forward direction. Since a low beam and a high beam are thrown in different directions, the light source (i.e., discharge light emitting portion or filament) is commonly differentiated between low and high beams. Also known are vehicle headlights using a single light source. This design is often inevitable with dual headlights using a discharge electric bulb as a light source bulb.
Vehicle headlights using a single light source are adapted to switch between a low beam and a high beam by mechanically displacing optical elements such as a light source bulb and a reflector and to this end they have a beam switching device. If the beam switching device is of such a type as to perform beam switching by displacing the light source bulb relative to the reflector, it is necessary that the light source be moved between specified positions while assuring high positional precision.
As shown in FIG. 10a, the ideal luminous intensity distribution pattern for a low beam satisfies the requirements for both providing good visibility in a forward direction for the driver and assuring anti-glare for the driver of an oncoming vehicle. FIG. 10b shows the ideal luminous intensity distribution pattern for a high beam that provides good visibility at far distance. In order to achieve this object using a reflector with the reflective surface of which is made of a paraboloid of revolution, it is usually the most effective to set the light source in low and high beam positions as shown in FIG. 11, where the high beam position Hp is backward and obliquely downward of the low beam position Lp along the optical axis Ax of the reflector. In FIG. 11, F designates the focal position of the paraboloid of revolution.
The same discussion holds with a step reflector the reflective surface of which is composed of a plurality of reflector elements and a reflector the reflective surface of which is composed of a free curved surface.
The above-described beam switching device which performs beam switching by merely displacing the light source bulb relative to the reflector has a difficulty in that if one wants to displace the light source bulb along the optical axis of the reflector and vertically relative to it, the bulb displacing mechanism for causing relative displacements of the light source bulb becomes complicated. As a further problem, the amounts of displacement of the light source bulb are set at specific values that are closely related to other optical elements such as the reflector and the lens and, hence, the bulb displacing mechanism must also have a dedicated structure adapted to these other optical elements. If the amounts of displacement of the light source bulb are to be altered on account of variations in the specifications of other optical elements, it is also necessary to use a new bulb displacing mechanism.
Consider, for example, the beam switching device shown in FIG. 12. To switch a low beam to a high beam, a bulb holder 144 of a light source bulb 18 is cam driven to be displaced linearly to a backward and obliquely downward position relative to a reflector 20 so that a light source 18a is moved parallel. A problem with this structure is its complexity since slide shafts 148 and 150 that compose a valve displacing mechanism 140 and the shaft inserting portion of the bulb holder 144 through which those shafts are supported have to be formed to extend downward at an angle. What is more, in order to change the relationship between the two light source positions Lp (low beam position) and Hp (high beam position), the angles of inclination of the shafts 148 and 150, and, hence, all related parts of the mechanism must be changed.
FIG. 13 shows another prior art beam switching device. To switch a low beam to a high beam, a bulb holder 144 of a light source bulb 18 is cam driven to pivot about a longitudinal axis D to move backward of a reflector 20 so that a light source 18a is moved backward and obliquely downward. In order to have the light source 18a positioned in either Lp or Hp by means of such a pivotal bulb displacing mechanism 140, the longitudinal axis D about which the bulb holder 144 pivots must be set on line L indicated by a one-long-and-two-short dashed line and this increases the size of the bulb displacing mechanism 140. As a further problem, if one wants to change the relationship between the two light source positions Lp and Hp, the position of the longitudinal axis D and, hence, all related parts of the mechanism must be changed.